A conventional, low-shrinkage, wet type unsaturated polyester (B.M.C) composition has been produced by the following process. First, an unsaturated polyester resin, a hardener, and a low shrinkage rendering agent, e.g., polystyrene powder and polyethylene powder, in a total amount of from 20 to 28% by weight (hereinafter, all percents are by weight) based on the amount of the composition to be produced, are mixed with from 1.2 to 2.0%, based on the amount of the composition, of additives including a mold release agent and lubricant, such as stearic acid, calcium stearate, and the like, and this mixture is stirred, during which a calcium carbonate powder as a filler is incorporated into the mixture in an amount of from 45 to 60% based on the amount of the composition. Several minutes later, a slight amount of a carbon powder is added as a colorant and the resulting mixture is blended in a batch-type mixer for 40 minutes. Glass fibers are then added as a reinforcement in an amount of from 25 to 28% based on the amount of the composition and finishing blending is then conducted for 5 to 10 minutes. In this process, the reinforcement is added lastly because degradation of glass fibers should be prevented as far as possible so as to impart sufficient strength to formed products to be obtained from the formulation. Further, in order to prevent a temperature increase of the mixture due to stirring or blending, the temperature of the mixture is regulated by means of a jacket.
By molding or otherwise forming the compound obtained by the above-described process, formed products are obtained which have a thermal conductivity of 1.8.times.10.sup.-3 cal/cm.sec..degree.C. due to the calcium carbonate particles contained therein in an amount of from 45 to 60%.
An application example for such a formed product obtained from the above low-shrinkage wet type unsaturated polyester (B.M.C) composition to the housing of earth-leakage circuit breaker is illustrated below.
In a prior known earth-leakage circuit breaker as shown in FIG. 2, the cover 1 and base 2, each being a resinous molded part, have sufficient thicknesses and have sufficient internal spaces between themselves and each of the conductive members, e.g., the front terminal board (not shown) and the rear terminal board 5, which have sectional areas suited to electric current to be applied. Because of this, even in this earth-leakage circuit breaker employing such formed parts having the above-described conventional composition, the Joule heat generated therein is readily dissipated throughout the circuit breaker, without leaving a local thermal stress, and the dissipated heat is radiated from the housing surface and other parts.
In known electrical devices or instruments having a housing, e.g., earth-leakage circuit breakers of the above-described type, the amount of Joule heat generated by the conductive circuit parts therein is increasing with the recent trend toward size reduction and functionality increase or toward breaker capacity increase. At present, the temperature increase problem for these electrical devices or instruments has reached such a level that heating of such devices or instruments cannot be coped with by the measures conventionally employed for temperature increase prevention, such as volume increase of conductive parts, utilization of spaces, heat insulation, etc. For this reason, it has become difficult to manufacture electrical devices or instruments that meet temperature increase standards provided for with respect to the respective products.
As one specific example of the above case, there is a desire to incorporate a ground fault detector having an integrated circuit into an earth-leakage circuit breaker of the above-described type without changing the outer dimensions (mounting size) of the breaker or reducing the mechanical strength of the circuit breaker housing and without lowering the current-conduction capacity of each conductive part.
In order to satisfy such a desire, components of the earth-leakage circuit breaker should be housed in the housing at an increased packaging density, so that the internal space within the circuit breaker becomes small and conductive members come to be disposed closer to one another.
As a result, if a formed part having the above-described conventional formulation is used as the housing, the following problems, for example, are posed. That is, thermal interaction between members changes the conditions under which the bimetal bends and prevents the integrated circuit and other members which are sensitive to thermal stress from performing their usual functions.
These problems are partly attributable to the insufficient thermal conductivity of the formed parts produced from the above-described low-shrinkage, wet type unsaturated polyester (B.M.C) composition and used in the housing, heat generating part-supporting parts, and the like. Illustratively stated, because of such formed parts, heat dissipation cannot occur sufficiently and, as a result, the heat generated is confined within the housing and the temperature therein rises unavoidably. In order to eliminate the above problems, it is necessary to improve the conventional, low-shrinkage, wet type unsaturated polyester (B.M.C) composition so as to give formed parts having high thermal conductivity and, hence, good heat-dissipating properties.